Add a default build pack

Gives you a blank python3 environment

repo2docker/app.py

@@ -26,7 +26,7 @@ import subprocess
 
 from .detectors import (
     BuildPack, PythonBuildPack, DockerBuildPack, LegacyBinderDockerBuildPack,
-    CondaBuildPack,
+    CondaBuildPack, DefaultBuildPack
 )
 from .utils import execute_cmd
 from . import __version__
@@ -93,7 +93,7 @@ class Repo2Docker(Application):
 
     buildpacks = List(
         Type(BuildPack),
-        [LegacyBinderDockerBuildPack, DockerBuildPack, CondaBuildPack, PythonBuildPack],
+        [LegacyBinderDockerBuildPack, DockerBuildPack, CondaBuildPack, PythonBuildPack, DefaultBuildPack],
         config=True,
         help="""
         Ordered list of BuildPacks to try to use to build a git repository.

repo2docker/detectors.py

@@ -158,3 +158,10 @@ class PythonBuildPack(S2IBuildPack):
                 pass
             self.build_image = self.runtime_builder_map[self.runtime]
             return True
+
+class DefaultBuildPack(S2IBuildPack):
+    build_image = Unicode('jupyterhub/singleuser-builder-venv-3.5:v0.2.0')
+    name = Unicode('default')
+    def detect(self, workdir):
+        return True
+